clench / clinch
To close tightly versus to settle or fasten.
To clench is to close tightly: clench a fist, clench your teeth. To clinch is to settle a matter, or to fasten: clinch the deal, clinch a nail. Do not use clench the deal.
Examples
UseHe clenched his jaw.
UseThat clinched the vote.
AvoidThat clenched the vote.